A gas well has casing with perforations or an open hole completion below the casing. Typically the gas well has a string of tubing with a packer located above the perforations, although in some wells, a packer is not employed. The gas flows from a gas producing zone up the tubing to the wellhead and into a pipeline.
A desired minimum pressure is required at the surface or wellhead for delivery into the gas pipeline. A pressure drop due to frictional losses occurs as the gas flows from the perforations up the tubing. In wells that have been partially depleted, the pressure at the producing zone may be insufficient to overcome the frictional pressure drop and still achieve the desired wellhead pressure.
Compressors are commonly used at the surface of low pressure gas wells for creating a negative pressure at the wellhead to enhance gas flow and for compressing the gas at the wellhead to achieve the desired wellhead pressure. The compressor may be a turbine type, a liquid ring type, or a screw pump. A screw pump has at least two rotors with helical profiles formed thereon. The helical profiles interleave each other. One of the rotors is driven, which causes the other to rotate. A screw pump is capable of pumping multi-phase fluids. Turbine type compressors are generally not capable of multi-phase production, thus for gas wells that produce a significant amount of liquid, the liquids are normally separated from the well fluid before reaching the turbine type compressor.
As an alternative to surface compressors, it has also been proposed to a connect a downhole motor to a turbine gas compressor and lower the assembly into a gas well for compressing the gas downhole. A variable speed power supply may be used at the surface to vary the speed of the motor. While these various systems are workable with low pressure gas wells, improvements in efficiency are desired.